The present invention relates to a multilayer parison extruder and method for extruding the multilayer parison.
A hollow resin receptacle is manufactured utilizing a blow molding method. Specifically, the hollow receptacle can be manufactured by blowing pressurized fluid through a die.
In a conventional multilayer parison extruder disclosed in the Japanese unexamined published patent application No. 62-99115, a ring member is concentrically provided with respect to an annular passage of a primary material resin and an auxiliary material resin is extruded from not less than one annular nozzle provided in the ring member. Each annular nozzle has an inlet port from which the material flows. In this structure, a material having good forming characteristics and being permeable to gasoline, such as a high density polyethylene, is used as a primary resin material and is utilized for most outer and inner layers. In contrast, a material which is impermeable to gasoline, such as a nylon resin, is used for an auxiliary material and is utilized for middle layers. The gaps between each layer are adhered by adhesive material layers. The thus produced multilayer parison can be used to produce a gasoline tank or the like.
However, the problem associated with a multilayer parison manufactured using the conventional multilayer parison extruder is that the thickness of each layer in the circumferential direction is not uniform. More specifically, a material flowing from the inlet port must be circumferentially spread and extruded uniformly. However, in the conventional structure, in the nozzle portion located proximate the inlet port, the extrusion pressure is relatively high, and conversely in the nozzle portion located away from the inlet port, the extrusion pressure is relatively low. Therefore, the flow rate of the material which is extruded from the nozzle located near the inlet port is larger than the flow rate of the material which is extruded from the nozzle located away from the inlet port. For this reason, the thickness of parison becomes relatively thick near the inlet port, and conversely, the thickness of parison becomes relatively thin at the portion away from the inlet port, thereby resulting in an uneven thickness in the circumferential direction. The uneven thickness results in the production of defective products.